The present application relates to an electrode with a porous protective film, a nonaqueous electrolyte secondary battery, and a method for manufacturing an electrode with a porous protective film. In particular, the present application relates to an electrode with a porous protective film, the electrode including an electrode, in which an active material layer is disposed on a collector, and a porous protective film, which is disposed on a surface of the active material layer and which contains fine particles, a binder, a surfactant, and a thickener, a nonaqueous electrolyte secondary battery, and a method for manufacturing an electrode with a porous protective film.
In recent years, enhancement of performance, miniaturization, and enhancement of portability of electronic equipment have progressed because of the advance of electronic technology. Consequently, requirements for increases in the energy densities of batteries used for such electronic equipment have become intensified.
As for secondary batteries used for such electronic equipment in the related art, aqueous electrolytic solution secondary batteries, e.g., nickel-cadmium batteries and lead batteries, have been used.
These secondary batteries exhibit low discharge voltages and, therefore, are unsatisfactory from the viewpoint of production of secondary batteries having high energy densities.
Hence, research and development on nonaqueous electrolytic solution secondary batteries as an alternative to the nickel-cadmium batteries and the like have been conducted recently.
Examples of the above-described nonaqueous electrolytic solution secondary batteries include a secondary battery in which a carbon material is used as a negative electrode active material, a lithium cobalt composite oxide is used as a positive electrode active material, and a nonaqueous electrolytic solution is prepared by dissolving a lithium salt into a nonaqueous solvent.
This secondary battery has advantages that the battery voltage is high and self discharge is at a low level, and thereby, a high energy density can be realized. In the case where the above-described carbon material and the lithium cobalt composite oxide are actually used as active materials, they are made into powders having average particle diameters of 5 to 50 μm, and the powders are dispersed into solvents together with binders, so as to prepare each of a negative electrode mix slurry and a positive electrode mix slurry. Subsequently, the individual slurries are applied to metal foil serving as respective collectors, so as to form a negative electrode active material layer and a positive electrode active material layer. A negative electrode and a positive electrode prepared by forming the negative electrode active material layer and the positive electrode active material layer, respectively, on the collectors are separated with a separator therebetween, and they are held in a battery can while being in that state.
Here, regarding the above-described nonaqueous electrolytic solution secondary battery, it should be noted that the electrical conductivity of a nonaqueous electrolytic solution used therefor is about two orders of magnitude smaller than the electrical conductivity of an aqueous electrolytic solution.
Therefore, it is desirable that the structure of the battery is made optimum for movement of an electrolyte.
Consequently, in the above-described nonaqueous electrolytic solution secondary battery, a very thin separator having a thickness of about 10 to 50 μm is used as a separator for separating the positive electrode and the negative electrode.
As described above, regarding the nonaqueous electrolytic solution secondary battery, an electrode is produced by applying a mix slurry containing an active material powder to a collector so as to form an active material layer and, thereafter, is held into a battery can.
At this time, the electrode provided with the active material layer passes through various steps, e.g., a step of laminating the electrode and a separator and a cutting step to cut into a predetermined electrode shape, until the electrode is held into the battery can.
However, during running of a raw electrode band in the lamination step and the cutting step, an inconvenience occurs in that the active material is dropped from the active material layer because of contact of the active material layer with a guide roller and the like and a part of the dropped active material adheres again to a surface of the electrode. Furthermore, in the step of holding the electrode into the battery can, an inconvenience occurs in that fine metal particles intrude into the battery. Moreover, the dropped active material, which adheres to the electrode surface again, and the fine metal particles intruded into the battery have particle diameters of 5 to 200 μm which are larger than or equal to the thickness of the separator. Consequently, a problem occurs in that they penetrate the separator in an assembled battery and cause physical internal short-circuit.
Accordingly, Japanese Unexamined Patent Application Publication No. 7-220759 proposes disposition of a porous protective film produced by applying a fine particle slurry containing a binder and fine particles to a surface of any one of the negative electrode active material layer and the positive electrode active material layer and conducting drying.
In addition, the air permeability of this porous protective film is selected so as to become about 680 sec/100 ml in the related art.